The combustion of organic materials are often free radical chain reactions, which can usually be summarised as the oxidation of carbon content of the material to form its oxides and the oxidation of hydrogen to form water.
The corrosion of metals is the result of the slow combustion of the metals with the oxygen in air.
Combustion of Metals
The chief product of the reaction is carbon dioxide, although a little carbon monoxide is also formed :
C + O2 ==> CO2 Carbon Dioxide 2C + O2 ==> CO Carbon MonoxideWhen shaken with water, the Carbon Dioxide, CO2, dissolves forming a solution of the very weak and unstable solution of Carbonic Acid, H2CO3, which changes the blue colour of litmus to a port-wine red colour :
CO2 + H2O ==> H2CO3 Carbon Carbonic Dioxide AcidIf another jar of containing Carbon Dioxide, CO2, from the combustion of carbon is shaken with lime water, this becomes milky from the formation of a precipitate of Calcium Carbonate, CaCO3.
Ca(OH)2 + CO2 ==> CaCO3 + H2O Calcium Calcium Hydroxide Carbonate
2 H2 + O2 ==> 2 H2O
Other metals do not burn readily in oxygen, but react with it on heating to form Oxides.
2 Na + O2 ==> Na2O2 Sodium PeroxideWhen this substance is dissolved in water, oxygen is evolved and an alkaline solutions, containing sodium hydroxide, which turn red litmus blue is formed.
2 Na2O2 + 2 H2O ==> 4 NaOH + O2 Sodium Sodium Peroxide Hydroxide
2 K + O2 ==> K2O2 Potassium Peroxide K + O2 ==> KO2 Potassium DioxideWhen these substances are dissolved in water, oxygen is evolved and an alkaline solutions, containing potassium hydroxide, which turn red litmus blue is formed.
> 2 K2O2 + 4 H2O ==> 4 KOH + O2 Potassium Potassium Peroxide Hydroxide 4 KO2 + 2H2O ==> 4 KOH + 3 O2 Potassium Potassium Dioxide Hydroxide
2 Ca + O2 ==> 2 CaO Calcium Oxide CaO + H2O ==> Ca(OH)2
3 Fe + 2 O2 ==> Fe3O4 Ferrosoferric Oxide
Ferrosoferric oxide may be regarded as a compound of ferrous oxide and ferric oxide and it is sometimes called a mixed oxide.
FeO + Fe2O3 ==> Fe3O4
The chemical characteristic of the non-metals is that their oxides on reaction with water give rise to acids. The acidic properties of the hydrolysis of these oxides vary considerable, so that the oxides of sulphur and phosphorus give rise to strong acids, while the oxides of boron give rise to weak acids.
Some non-metallic elements, such as phosphorus, sulphur, and carbon, burn in oxygen to form Acidic Oxides
P4 + 5 O2 ==> 2 P2O5When water is poured into the jar the oxide dissolves and phosphoric acid is formed, which changes the colour of blue litmus solution to red :
P2O5 + 3 H2O ==> 2 H3PO4
S + O2 ==> SO2 Sulphur Dioxide 2 S + 3 O2 ==> 2 SO3 Sulphur Trioxide
When shaken with water, the products of combustion dissolve, forming an acidic solution which turns litmus red.
SO2 + H2O ==> H2SO3 Sulphur Sulphurous Dioxide Acid SO2 + H2O ==> H2SO4 Sulphur Sulphuric Trioxide Acid